Python has a handy language feature called "for-else" (similarly, "while-else"), which looks like this:
for obj in my_list:
if obj == target:
break
else: # note: this else is attached to the for, not the if
print "nothing matched", target, "in the list"
Essentially, the else is skipped if the loop breaks, but runs if the loop exited via a condition failure (for while) or the end of iteration (for for).
Is there a way to do this in bash? The closest I can think of is to use a flag variable:
flag=false
for i in x y z; do
if [ condition $i ]; then
flag=true
break
fi
done
if ! $flag; then
echo "nothing in the list fulfilled the condition"
fi
which is rather more verbose.
Using a subshell:
( for i in x y z; do
[ condition $i ] && echo "Condition $i true" && exit;
done ) && echo "Found a match" || echo "Didn't find a match"
You could put a sentinel value in the loop list:
for i in x y z 'end-of-loop'; do
if [ condition $i ]; then
# loop code goes here
break
fi
if [ $i == 'end-of-loop' ]; then
# your else code goes here
fi
done
Something very hacky to introduce similar syntax:
#!/bin/bash
shopt -s expand_aliases
alias for='_broken=0; for'
alias break='{ _broken=1; break; }'
alias forelse='done; while ((_broken==0)); do _broken=1;'
for x in a b c; do
[ "$x" = "$1" ] && break
forelse
echo "nothing matched"
done
$ ./t.sh a
$ ./t.sh d
nothing matched
You can do this but I personally find it hard to read:
while :;
do for i in x y z; do
if [[ condition ]]; then
# do something
break 2
done
echo Nothing matched the condition
break
done
I also enjoy devnull's answer, but this is even more pythonic:
for i in x y z; do
[ condition $i ] && break #and do stuff prior to break maybe?
done || echo "nothing matched"
This will only echo "nothing matched" if the loop did not break.
You can change this
if ! $flag; then
echo "nothing in the list fulfilled the condition"
fi
to something simpler like this
"$flag" || echo "nothing in the list fulfilled the condition"
if you only have one statement after it, although that's not really going to help much.
Related
I think i'm missing something very obvious. But shouldn't the following code produce the opposite response? I thought if the statement "if s == d" is used and s is not equal to d then the if statement should return false and not run the following code. This is not what appears to happen. Can anyone explain what i've missed. I think it's something very obvious.
Thanks
s=2
d=3
if ! [ "$s == $d" ]; then echo "hello"; fi
if [ "$s == $d" ]; then echo "hello"; fi
hello
You are quoting the entire string "$s == $d" when you should be quoting the two arguments "$s" and "$d".
This means that instead of comparing $s to $d, you are checking whether "2 == 3" is a non-empty string (which it is).
This will correctly print "not equal":
s=2
d=3
if ! [ "$s" == "$d" ]; then echo "not equal"; fi
if [ "$s" == "$d" ]; then echo "equal"; fi
I am attempting to run a block of code if one flag is set to true and the other is set to false. ie
var1=true
var2=false
if [[ $var1 && ! $var2 ]]; then var2="something"; fi
Since that did not evaluate the way that I expected I wrote several other test cases and I am having a hard time understanding how they are being evaluated.
aa=true
bb=false
cc="python"
if [[ "$aa" ]]; then echo "Test0" ; fi
if [[ "$bb" ]]; then echo "Test0.1" ; fi
if [[ !"$aa" ]]; then echo "Test0.2" ; fi
if [[ ! "$aa" ]]; then echo "Test0.3" ; fi
if [[ "$aa" && ! "$bb" ]]; then echo "Test1" ; fi
if [[ "$aa" && ! "$aa" ]]; then echo "Test2" ; fi
if [[ "$aa" ]] && ! [[ "$bb" ]]; then echo "test3" ; fi
if [[ "$aa" ]] && ! [[ "$cc" ]]; then echo "test4" ; fi
if [[ $aa && ! $bb ]]; then echo "Test5" ; fi
if [[ $aa && ! $aa ]]; then echo "Test6" ; fi
if [[ $aa ]] && ! [[ $bb ]]; then echo "test7" ; fi
if [[ $aa ]] && ! [[ $cc ]]; then echo "test8" ; fi
When I run the preceding codeblock the only output I get is
Test0
Test0.1
Test0.2
however, my expectation is that I would get
Test0
Test1
Test3
Test5
Test7
I have tried to understand the best way to run similar tests, however most examples I have found are set up in the format of
if [[ "$aa" == true ]];
which is not quite what I want to do. So my question is what is the best way to make comparisons like this, and why do several of the test cases that I would expect to pass simply not?
Thank you!
Without any operators, [[ only checks if the variable is empty. If it is, then it is considered false, otherwise it is considered true. The contents of the variables do not matter.
Your understanding of booleans in shell context is incorrect.
var1=true
var2=false
Both the above variables are true since those are non-empty strings.
You could instead make use of arithmetic context:
$ a=1
$ b=0
$ ((a==1 && b==0)) && echo y
y
$ ((a==0 && b==0)) && echo y
$
$ ((a && !(b))) && echo y; # This seems to be analogous to what you were attempting
y
The shell does not have Boolean variables, per se. However, there are commands named true and false whose exit statuses are 0 and 1, respectively, and so can be used similarly to Boolean values.
var1=true
var2=false
if $var1 && ! $var2; then var2="something"; fi
The difference is that instead of testing if var1 is set to a true value, you expand it to the name of a command, which runs and succeeds. Likewise, var2 is expanded to a command name which runs and fails, but because it is prefixed with ! the exit status is inverted to indicate success.
(Note that unlike most programming languages, an exit status of 0 indicates success because while most commands have 1 way to succeed, there are many different ways they could fail, so different non-zero values can be assigned different meanings.)
true and false are evaluated as strings ;)
[[ $var ]] is an equivalent of [[ -n $var ]] that check if $var is empty or not.
Then, no need to quote your variables inside [[. See this reminder.
Finally, here is an explication of the difference between && inside brackets and outside.
The closest you can come seems to be use functions instead of variables because you can use their return status in conditionals.
$ var1() { return 0; }
$ var2() { return 1; } # !0 = failure ~ false
and we can test this way
$ var1 && echo "it's true" || echo "it's false"
it's true
$ var2 && echo "it's true" || echo "it's false"
it's false
or this way
$ if var1; then echo "it's true"; else echo "it's false"; fi
it's true
$ if var2; then echo "it's true"; else echo "it's false"; fi
it's false
Hope this helps.
Here is my code:
#!/bin/bash
echo "Letter:"
read a
if [ $a = "a" ]
then
echo "LOL"
fi
if [ $a = "b" ]
then
echo "ROFL"
fi
Is there a way for me to loop this so that, after displaying either LOL or ROFL, I would be asked for a letter again?
Yes.
Oh, you want to know how?
while true; do
echo "Letter:"
read a
if [ $a = "a" ]
then
echo "LOL"
elif [ $a = "b" ]
then
echo "ROFL"
fi
done
Of course, you probably want some way to get out of that infinite loop. The command to run in that case is break. I would write the whole thing like this:
while read -p Letter: a; do
case "$a" in
a) echo LOL;;
b) echo ROFL;;
q) break;;
esac
done
which lets you exit the loop either by entering 'q' or generating end-of-file (control-D).
Don't forget that you always want -r flag with read.
Also there is a quoting error on that line:
if [ $a = "a" ] # this will fail if a='*'
So here is a bit better version(I've also limited the user to input only 1 character):
#!/bin/bash
while true; do
read -rn1 -p 'Letter: ' a
echo
if [[ $a = 'a' ]]; then
echo "LOL"
elif [[ $a = 'b' ]]; then
echo "ROFL"
else
break
fi
done
Or with switch statement:
#!/bin/bash
while read -rn1 -p 'Letter: ' a; do
echo
case $a in
a) echo LOL;;
b) echo ROFL;;
*) break;;
esac
done
I have a variable named l and it contains a string of characters
for example
echo $l output will be a1,a2,a3,a4,a5,a6,a7,a8,b1,b2,b3
How do i search for b1 and store it in a new temporary that will be used for condition checking. I cant use awk as the positioning will changed everything.
So my theory will be something like this, getting b1 from that string and i will stored in variable J
$j = b1
if [ $j = 'b1' ]
then
echo "This is a true statment by b1"
else
echo "This is a false statement not by b1"
fi
Is there a faster way to do this as i think i am duplicating the steps.
Thanks
I think this might work for you:
l="a1,a2,a3,a4,a5,a6,a7,a8,b1,b2,b3"
j="b1"
if [[ $l =~ $j ]]; then
echo "This is a true statement by $j"
else
echo "This is a false statement by $j"
fi
or
[[ $l =~ $j ]] && echo "This is a true by $j" || echo "This is false by $j"
I have a shell script that I want to ask the user if they want to continue. If they type 'n' and press enter the script will exit.
If they press 'y' and enter it will continue to run. I have this at the top of my script but it continues regardless of what I type.
What am I doing wrong ?
goon=
while [ -z $goon ]
do
echo -n 'Do you want to continue? '
read goon
if [[ $goon = 'n' ]]
then
break
fi
goon=
done
Use an infinity loop and case/esac like this:
while true
do
read -r -p 'Do you want to continue? ' choice
case "$choice" in
n|N) break;;
y|Y) echo 'Do your stuff here';;
*) echo 'Response not valid';;
esac
done
The 'break' statement will exit you out of your while loop.
If you want to exit the script you want to use 'exit'.
That works perfectly well for me if I get rid of the doubled square brackets:
if [ $goon = 'n' ]
Rather than echo + read, just use read -p
read -p "Do you want to continue? " goon
Here's a working example (== instead of = for equality testing)
goon=
while [ -z $goon ]
do
echo -n 'Do you want to continue? '
read goon
if [[ $goon == 'n' ]]
then
break
fi
goon=
done
Strange thing, the original works ok for me too ...
I'm less than positive, but it looks as though your if statement will always evaluate to false.
Here's a resource on BASH coding that explains how to use conditionals in the way you are attempting to.
http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-6.html#ss6.4
I think you mean "exit" instead of "break", if you want the whole script to exit.
Also, you aren't actually checking for "y", so it loops forever even if they do say "y".
if [[ $goon = 'n' ]]
then
exit
fi
if [[ $goon = 'y' ]]
then
break
fi
Try below script
#!/bin/bash
pause ()
{
REPLY=X
while [ "$REPLY" == "X" ] || [ "$REPLY" != "n" ]
do
echo -e "\t\tPress 'n' to continue\t\t\tPress 'x' to quit"
read -n1 -s
case "$REPLY" in
"x") exit ;;
"X") echo "case sensitive!!" ;;
"n") clear ;;
"N") echo "case sensitive!!" ;;
* ) echo "Invalid Option" ;;
esac
done
}
pause
echo "Hi"
am sure this script will give you only two options to move around...